It seems I left out an important detail... hopefully I didn't cause you too much frustration. There is a little bug in Meshlab when scaling. If you don't Go to Normals->Freeze Current Matrix, the scaling won't be applied, and your model will still be so tiny it will cause a manifold error.

I just repeated the steps I gave you, scaled it, then used the cryptic sounding "Freeze Current Matrix" (which just means "Apply the Filter" in English), and it worked both times I tried it. I'm not sure how big you wanted it to be, but scaling 1000x (not 100x) and uploading as millimeters gave me an object 40mm wide that cost $9 in WSF material. Scaling by 100x and uplaoding as inches gave an object 100mm wide that cost $125 in WSF.

We do want to help you figure out what is going wrong and not just tell you what to click on. Here is the best explanation I can come up with of what happened:

Your spline had an extra segment, so your spin was not closed. This confused Blenders export filter and made a mess of your normals. Blenders default unit is meters, so the model was extremely small. After scaling correctly to convert the units and fixing the normals, the "Merge Close Vertices" filter closed the mesh, but left some junk from the extra spline segment inside the model (shown in the screenshot). Shapeways will accept the model with the extra geometry still inside because the software removes it automatically (called "Merging" or "Unifying").

The best solution is of course to learn these potential problems well enough to avoid them while modeling, but that is easier said than done, so I think this Meshlab approach is a good one since it is fully automatic and doesn't require you to select or manually edit anything. Meshlab can be very touchy though, I admit. I hope that clarified things a little!